Multilayer Conductive Hybrid Nanosheets as Versatile Hybridization Matrices for Optimizing the Defect Structure, Structural Ordering, and Energy‐Functionality of Nanostructured Materials

Abstract The hybridization of conductive nanospecies has garnered significant research interest because of its high efficacy in improving the diverse functionalities of nanostructured materials. In this study, a novel synthetic strategy is developed to optimize the defect structure, structural ordering, and energy‐related functionality of nanostructured‐materials by employing a multilayer multicomponent two‐dimenstional (2D) graphene/metal oxide/graphene nanosheet (NS) as a versatile hybridization matrix. The hybridization of the robust trilayer, polydiallyldiammonium (PDDA)‐anchored reduced‐graphene oxide (prGO)/metal oxide/prGO NS effectively enhance the structural ordering and porosity of the hybridized MoS2/MnO2 NS through suppression of defect formation and tight stacking. In comparison with monolayer rGO/RuO2 NS‐based homologs, the 2D superlattice trilayer prGO/RuO2/prGO NS hybrids deliver better functionalities as a hydrogen evolution electrocatalyst and as a supercapacitor electrode, demonstrating the merits of hybridization with multilayer NSs. The advantages of using multilayer multicomponent conductive NSs as hybridization matrices arise from the enhancement of charge and mass transport through the layer flattening or defect suppression of the hybridized NSs and the increase in porosity, as evidenced by density functional theory calculations. Finally, the universal utility of multilayer NSs is confirmed by investigating the strong effect of the stacking order on the electrocatalytic functionality of MoS2/rGO/RuO2 films fabricated through layer‐by‐layer deposition.

hybridization on their crystal structures. The crystal shapes and hybrid structures of the present materials were probed with high-resolution transmission electron microscopy (HR-TEM) (Jeol JEM-2100F microscope with an accelerating voltage of 200 kV) and fieldemission scanning electron microscopy (FE-SEM) (Jeol JSM-6700F microscope). The elemental distributions of the present materials were probed with energy dispersive spectrometry (EDS)elemental mapping analysis. The evolution of porosity upon the hybridization was studied by N 2 adsorptiondesorption isotherm analysis at 77 K using Micromeritics ASAP 2020. Mo K-edge, Mn K-edge, and Ru K-edge X-ray absorption spectra (XAS) were measured to probe the oxidation state and local symmetry of the present materials. All the present XAS data were obtained at the beam lines 8C and 10C of Pohang Accelerator Laboratory (PAL, Pohang) in Korea. The XAS measurements were carried out at room temperature using gas-ionization detectors. The XAS analysis was done according to the standard procedure, as reported previously. [6] The chemical bonding characters of the present nanohybrids were investigated with X-ray photoelectron spectroscopy (XPS) measurement (Thermo VG, UK). All the XPS data were energy-referenced to the adventitious Au 4f peak (BE = 84 eV). Micro-Raman spectroscopic analysis was carried out for the present materials using Horiba Jobin-Yvon LabRam Aramis spectrometer, in which Ar + ion laser with a wavelength of 514 nm was used as the excitation source.
Computational Details: Spin-polarized density functional theory (DFT) calculations were performed using Vienna Ab-initio Simulation Package (VASP) code. [7] The exchangecorrelation energy was described using Perdew-Burke-Ernzerhof(PBE) functional [8] and the van der Waals interaction was corrected using zero-damping Grimme-D3 method. [9] The reciprocal space was sampled using Monkhorst-Pack [10] grid of 4  2  1, and core electrons were treated by projector augmented wave (PAW) method. [11] The sulfur monovacant MoS 2 was modeled using 4  4 supercell of 1T′-MoS 2 , where one S atom was removed to evaluate the S vacancy formation energy. The chemical potential of S was calculated using S 8 molecule.
Electrocatalytic Activity Measurement: For the test of hydrogen evolution reaction (HER) activity, the linear sweep voltametry (LSV) curves of the present materials were measured using an electrochemical working station with a three-electrode system, where Pt wire and saturated calomel electrode (SCE) (sat. KCl) were used as a counter and a reference electrode, respectively. To fabricate the working electrode, the electrode ink was prepared by dispersing the active material (2.5 mg) in the mixture of Milli-Q water, isopropyl alcohol, and 5wt% Nafion solution under sonication. 10 L of the obtained ink was deposited on glassy carbon and dried in oven. 1 M aqueous KOH solution was used as electrolyte after the purging of N 2 gas for 30 min. The LSV data were collected in the potential region from 0.95 to 1.35 V (vs. SCE) at a scan rate of 5 mV sec 1 . During the electrochemical measurement, the working electrode was rotated at 1600 rpm to remove H 2 bubbles attached on the electrode surface.
The electrochemical impedance spectroscopy (EIS) data of the present materials were measured at several potentials with a frequency range of 0.110000 Hz. The scan-ratedependent charging current density was monitored by measuring cyclic voltametry (CV) curves at open circuit voltage (OCV). The electrochemical active surface areas (ECSAs) of the present materials were calculated from double-layer capacitance (C dl ) divided by specific capacitance (C s ). The C dl values were determined from the slope of charging current vs. scan rate measured by CV under the assumption that C s for flat surface in alkaline media is 40 F cm 2 . [12] Layer-by-layer (LbL) Film Preparation: The LbL hybrid films composed of MoS 2 , RuO 2 , prGO, and rGO NSs were deposited on the substrate of flurorine-doped tin oxide (FTO) and quartz glass. [13] Prior to film deposition, FTO glass was cleaned via the sonication under ethanol/acetone/distilled water and quartz glass was washed according to the previously-reported process. [13] For the first step of the LbL deposition, the FTO/qaurtz glass was precoated by immersing it in an aqueous solution of PEI (2.5 mg mL 1 ) at pH 9 for 30 min with the surface area of 1.5 1.5 cm 2 , resulting in the positively-charged surface state. After being rinsed gently with distilled water, the PEI-coated substrate was dipped into dialized suspension of MoS 2 NS (0.08 mg mL 1 ) for 20 min, followed by washing with distilled water.
Next, FTO glass coated with negatively-charged MoS 2 NS was consecutively dipped into 20 ml of prGO NS suspension (0.08 mg mL 1 ) for 20 min, followed by washing with distilled water. After coating positvely-charged prGO NS, negatively-charged MoS 2 or RuO 2 NSs were sequentially deposited with the same procedure. In each step, 20 mL of fresh colloidal suspension of dialyzed NSs (0.08 mg mL 1 ) was provided, followed by washing with fresh                           [29] For the creation of sulfur vacancy, MoS 2 colloid was reacted with H 2 O 2 solution (1 mol L 1 ) for 180 (1)/60 (2) sec.